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U.S. officials are expressing "high confidence" that Syria carried out last week's chemical weapons attack in the town of Douma, promising the West will respond and that all options remain on the table.

Despite Tweets from U.S. President Donald Trump earlier this week suggesting a missile strike might be imminent, the White House, State Department, and Pentagon have been cautious in their language, emphasizing the need to forge an airtight case against the government of Syrian President Bashar al Assad.

By late Friday, there were indications such a case was coming together.

"We have a very high confidence that Syria was responsible," White House Press Secretary Sarah Sanders told reporters at a briefing.

"We also hold Russia responsible for their failure to stop chemical weapons attacks from taking place," Sanders added. "Russia's failure to stop them and their continued [inaction] on this front has been part of the problem."

The State Department went one step further, claiming the U.S. now has definitive proof.

"We can say that the Syrian government was behind this attack," said spokeswoman Heather Nauert.

"We know that this was a chemical weapon that was used in Syria," Nauert said. "The kind or the mix, that we are still looking into."

Despite such confidence, other U.S. officials remained wary, warning while Syria's use of chemical weapons cannot be tolerated, much more is at stake given the backing it gets from Moscow.

"This is a chess game and the Russians are ratcheting up the pressure," a U.S. official told VOA on the condition of anonymity given the sensitivity of the situation. "They're playing dirty," the official added. "We need to think two or three steps ahead."

Russia, long an ally of Syrian President Bashar al-Assad, has warned a military conflict could erupt if the U.S. and its Western allies take action in Syria.

"We hope that ... the U.S. and their allies will refrain from military action against a sovereign state," Russian Ambassador to the U.N. Vassily Nebenzia said Friday.

Also Friday, Moscow alleged it had "proof that testifies to the direct participation of Britain" in staging the chemical attack in Douma.

Britain pressured the White Helmets, volunteers and activists who serve as first responders in rebel-held areas, to stage the attack, Defense Ministry spokesman Igor Konashenkov said.

Britain called the allegation a "blatant lie."